RC Car Active Power Distribution
I am also planning to bring this monster back to life -

Its two RC Cars joined together to make a twin motor 4wd car with independent throttle control to the front and rear axles. The result is a small car with way too much power, its very much in the spirit of a Group B rally car or the twin motored Suzuki Escudo rally car.

Saturday, December 1, 2012

This post provides a quick introduction to Direct Digital Synthesis (DDS) on the Arduino Due. The sample sketch outputs a sinewave at a 44.1Khz sample rate with 12 bit resolution using the built in Digital To Analog Converter (DAC).

Direct Digital Synthesis - Introduction
DDS is a common approach to generating sinewave output from digital systems. The approach is based around outputting samples from a description of the required waveform held in memory. The description is an array of samples which can be traversed to plot the waveform onscreen or by using DDS can generate a sinewave output at a selected frequency.

The 600 sample 12 Bit sine wave table used in the sketch below plotted on screen using processing -

The 128 sample 8 Bit wave tables used in the Illutron B synth -

Direct Digital Synthesis - Sample Rate
To generate an output waveform we need a way to regularly update the output value, we do this by using a timer interrupt. The frequency of this interrupt is termed the sample rate, in our case we are using a 44.1Khz sample rate meaning that we have an interrupt triggering 44,100 times per second.

As this interrupt is triggering at such a high rate it is very important that we keep it efficient, this is the reason for using a pre computed wavetable rather than computing the sine samples on the fly.

Direct Digital Synthesis - Frequency Generation
We can generate different frequencies by changing the rate at which we traverse through the wave table. One pass through the wavetable provides one complete cycle of the selected waveform (sine in our case). If we pass through the table once in 44,100 interrupts we generate a frequency of 1Hz, if we pass through 100 times faster, we get a sinewave output at a frequency of 100Hz.

Direct Digital Synthesis - Implementation and Terminology
The following terms are commonly used in reference to DDS and variations are used throughout the RC Arduino audio projects.

1) Phase Accumulator - this is an array index which points to the current sample in the selected wavetable.
2) Phase Increment - this value is used to advance the Phase Accumulator each time the interrupt is called.

Example usage of the phase accumulator used inside the timer interrupt to select the next sample in the wave table array -

// get the current sample from the sine table using the phase accumulator as the index
uint32_t ulOutput = nSineTable[ulPhaseAccumulator>>20];

Direct Digital Synthesis - Fixed Point Maths
To generate a full range of frequencies we will often need to use fractional values for our phase accumulator.

The usual method of working with fractional values is to use the float data type however in DDS we need our interrupts to run as fast as possible. With very few exceptions microcontrollers are not able to process the float data type at high speed, the solution is to use fixed point integer maths.

Fixed Points For Higher Integer Precision
There are many applications of fixed point arithmetic but in DDS the main use is to provide increased precision - equivalent to having many decimal points while still using the high speed integer (whole number) data type.

To get extra precision we use more bits than we need to represent our data range. For example the sample code below has a wave table size of 600 samples so the range of we need for our phase accumulator is 0 to 599.

We can represent this range with 10 bits but if we use 16 bits we have 6 additional bits of precession.

16 Bit Integer

Bit

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Value

32768

16384

8192

4096

2048

1024

512

256

128

64

32

16

8

4

2

1

Fixed
Point 10.6 using 16 Bit Integer

Bit

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Value

1024

512

256

128

64

32

16

8

4

2

1

0.5

0.25

0.125

0.0625

0.0313

We use this additional precision to perform fractional addition to the phase accumulator, the 10.6 format gives 1/32 precision as the least significant bit.

This line of code is actually adding two fixed point numbers, part of the trick is that the compiler does not know this, it sees them simply as two integers which it processes using very fast hardware integer addition.

When we want to use the phase accumulator as the array index, we perform an arithmetic shift six places to the right, this drops off the six bits of additional precession and leaves us with a 10 bit integer value for the array index.

// drop the six bits of additional precision and access the remaining 10 bits as an integer array index in the range 0 - 599

uint16_t unOutputValue = sineTable[ulPhaseAccumulator>>6];

The only trick to fixed point maths is to figure out a scheme which will work for your project and stick to it. Remember the compiler does not know that you are treating part of the number as fractions so the responsibility is on you the programmer to keep consistency.

Fixed Point In The RCArduino DDS Example Sketch
The standard data type for the 32 bit Arduino Due is a 32 bit integer, this gives us scope for very high levels of precession in integer maths. The scheme used for the phase increment and phase accumulator variables in the sample sketch is 12.20 (12+20 = 32 Bits) this can be seen in the following line of code where the precision is shifted out to access the pure 12 bit integer value -

uint32_t ulOutput = nSineTable[ulPhaseAccumulator>>20];

Where ever you see code that makes extensive use of '>>' operations you might be looking at fixed point arithmetic, in a follow up post we will look at how we perform fast fixed point multiplication. This is another key to fast and flexible synthesizer engines.

Fixed Point Multiplication is used in Audio synthesis to apply envelopes and amplitude modulation.

Envelope Applied to a DDS sine wave using Fixed Point Maths in the Illutron B Project -

DDS Sinewave for Arduino Due - The Sketch
Use a potentiometer connected to analog pin 0 to control the pitch. To hear the output you can use an amplifier circuit such as this one used throughout RCArduino - http://rcarduino.blogspot.com/2012/08/adding-audio-to-arduino-projects.html
The sinewave is output through DAC0 - Note that the DAC does not provide a full 0-3.3volt swing, this is a hardware limitation of the SAM3X8E chip.

Caution : The SAM3X8E microcontroller at the
heart of the Arduino Due is less able to sink and source current than
the AVR family of chips used in the 8-Bit Arduinos. See the update below for the most recent recommendations -

Update 09/06/2013 - Gaétan Ro has taken things much further in the latest build of his Groovuino project. The Project uses the technique described here combined with some sensors, a sequencer and filter to produce a really nice sounding groovebox. I plan to build one of these for myself and hope to be able to help Gaétan with its further development.

Update 28/03/2013 : There is little information on the SAM3X8E DAC in the datasheet however Arduino forum used 'stimmer' has found recommendations on a closely related chip which suggest that a 2K Ohm resistor should be placed as a series current limiting resistor between the DAC Output and the load. This assumes that the load has no resistance of its own which will be the case if you accidentally short something, if you know the resistance of your load, you can reduce this value, if not, its a reasonable starting point and will protect against accidents. For more information see the original thread here - http://arduino.cc/forum/index.php/topic,139733.15.html

Update 28/03/2013 : Gaétan Ro has been building on the techniques here to produce an Arduino Due based groove box he is calling the 'Groovuino'. You can hear a clip of the project in action here -

// Create a table to hold pre computed sinewave, the table has a resolution of 600 samples
#define WAVE_SAMPLES 600
// default int is 32 bit, in most cases its best to use uint32_t but for large arrays its better to use smaller
// data types if possible, here we are storing 12 bit samples in 16 bit ints
uint16_t nSineTable[WAVE_SAMPLES];

// if the phase accumulator over flows - we have been through one cycle at the current pitch,
// now we need to reset the grains ready for our next cycle
if(ulPhaseAccumulator > SAMPLES_PER_CYCLE_FIXEDPOINT)
{
// DB 02/Jan/2012 - carry the remainder of the phase accumulator
ulPhaseAccumulator -= SAMPLES_PER_CYCLE_FIXEDPOINT;
}

// get the current sample
uint32_t ulOutput = nSineTable[ulPhaseAccumulator>>20];

// we cheated and user analogWrite to enable the dac, but here we want to be fast so
// write directly
dacc_write_conversion_data(DACC_INTERFACE, ulOutput);
}

The showcase is based on the simplest synth engine I could create - 3 counters counting up at a rate controlled by three analog inputs.

It might sound simple but its a surprisingly rich sounding synth engine, here is how it works -

Two of the counters generate ramp waveforms - think about it, they are counting up from 0 to 4095, when they overflow, they go back to 0 and start the count again.

Example Ramp Output -

Example Ramp At A Higher Frequency -

These ramp waveforms are summed together at the output to generate a more complex waveform - two ramp waves of independent frequency superimposed on each other.

Example - Two triangle at frequency F and 3F mixed together to create a new output waveform.

The third ramp waveform is used to control the pitch. It is not mixed with the output waveforms instead it achieves pitch control by resetting the first two waveforms.

Output waveform reset at frequency determined by third counter - notice that the counter is not directly present in the output but controls the repetition/synchronization of the output which in turn creates the pitch.

To hear the technique in action connect three potentiometers to your Arduino Due on analog inputs 0,1,2 and an audio amplifier to DAC0.

Caution : The SAM3X8E microcontroller at the heart of the Arduino Due is less able to sink and source current than the AVR family of chips used in the 8-Bit Arduinos. I would suggest using a series resistor of around 500 Ohms when connecting the Arduino Due DAC to an external device. See this thread on the Arduino forum for more information - http://arduino.cc/forum/index.php/topic,139733.0.html

// RCArduino Quick And Dirty Synth for Arduino Due// RCArduino Quick And Dirty Synth by RCArduino is licensed under a Creative Commons Attribution 3.0 Unported License.// Based on a work at rcarduino.blogspot.com.// For helpful background information on Arduino Due Timer Configuration, refer to the following link// thanks to Sebastian Vik// http://arduino.cc/forum/index.php?action=post;topic=130423.15;num_replies=20// The following folders within the arduino install provide access to source code and documentation// its very low level, the documentation is next to useless, but it might help someone// C:\arduino-1.5.1r2\hardware\arduino\sam\system\libsam\source// C:/arduino-1.5.1r2/hardware/arduino/sam/system/CMSIS/Device/ATMEL/sam3xa/html/tc1.html// C:/arduino-1.5.1r2/hardware/arduino/sam/system/CMSIS/Device/ATMEL/sam3xa/html/SAM3XA.html// C:\arduino-1.5.1r2\hardware\arduino\sam\system\CMSIS\CMSIS\Documentation// These are the clock frequencies available to the timers /2,/8,/32,/128// 84Mhz/2 = 42.000 MHz// 84Mhz/8 = 10.500 MHz// 84Mhz/32 = 2.625 MHz// 84Mhz/128 = 656.250 KHz// // 44.1Khz = CD Sample Rate// Lets aim for as close to the CD Sample Rate as we can get -//// 42Mhz/44.1Khz = 952.38// 10.5Mhz/44.1Khz = 238.09 // best fit divide by 8 = TIMER_CLOCK2 and 238 ticks per sample // 2.625Hmz/44.1Khz = 59.5// 656Khz/44.1Khz = 14.88// 84Mhz/44.1Khz = 1904 instructions per tick// These variables represent our synth engine, its similar to a vastly simplified Auduino and has a similar sound.// Phase Accumulator controls the main pitch, grain1 and grain two phase accumulator control the pitch for the two grains// The grainPhaseAccumulators are essentially simple counters, when you count up and overflow back to 0, you generate a ramp waveform// we generate two and mix them together.uint32_t ulPhaseAccumulator = 0; // 32 bit phase accumulator, if we shift >> 20 bits we get a 12 bit value for our output ADCvolatile uint32_t ulPhaseIncrement = 0; // 32 bit phase increment, see belowuint32_t ulGrain1PhaseAccumulator = 0; // 32 bit phase accumulator, if we shift >> 20 bits we get a 12 bit value for our output ADCvolatile uint32_t ulGrain1PhaseIncrement = 0; // 32 bit phase increment, see belowuint32_t ulGrain2PhaseAccumulator = 0; // 32 bit phase accumulator, if we shift >> 20 bits we get a 12 bit value for our output ADCvolatile uint32_t ulGrain2PhaseIncrement = 0; // 32 bit phase increment, see below// full waveform = 0 to 4294967296// Phase Increment for 1 Hz =(4294967296/44100) = 1Hz// Phase Increment for frequency F = (4294967296/44100)*F#define SAMPLE_RATE 44100.0#define TICKS_PER_CYCLE (4294967296.0/SAMPLE_RATE)// Create a table to hold the phase increments we need to generate midi note frequencies at our 44.1Khz sample rate#define MIDI_NOTES 128uint32_t nMidiPhaseIncrement[MIDI_NOTES];void createNoteTable(float fSampleRate){ for(uint32_t unMidiNote = 0;unMidiNote < MIDI_NOTES;unMidiNote++) { // Correct calculation for frequency Serial.print(unMidiNote); Serial.print(" "); float fFrequency = ((pow(2.0,(unMidiNote-69.0)/12.0)) * 440.0); Serial.print(fFrequency); Serial.print(" "); nMidiPhaseIncrement[unMidiNote] = fFrequency*TICKS_PER_CYCLE; Serial.println(nMidiPhaseIncrement[unMidiNote]); }}void setup(){ Serial.begin(9600); createNoteTable(SAMPLE_RATE); /* turn on the timer clock in the power management controller */ pmc_set_writeprotect(false); pmc_enable_periph_clk(ID_TC4); /* we want wavesel 01 with RC */ TC_Configure(/* clock */TC1,/* channel */1, TC_CMR_WAVE | TC_CMR_WAVSEL_UP_RC | TC_CMR_TCCLKS_TIMER_CLOCK2); TC_SetRC(TC1, 1, 238); // sets <> 44.1 Khz interrupt rate TC_Start(TC1, 1); // enable timer interrupts on the timer TC1->TC_CHANNEL[1].TC_IER=TC_IER_CPCS; TC1->TC_CHANNEL[1].TC_IDR=~TC_IER_CPCS; /* Enable the interrupt in the nested vector interrupt controller */ /* TC4_IRQn where 4 is the timer number * timer channels (3) + the channel number (=(1*3)+1) for timer1 channel1 */ NVIC_EnableIRQ(TC4_IRQn); // this is a cheat - enable the DAC analogWrite(DAC0,0);}void loop(){ // read analog input 0 drop the range from 0-1024 to 0-127 with a right shift 3 places, // then look up the phaseIncrement required to generate the note in our nMidiPhaseIncrement table uint32_t ulInput = analogRead(0); ulPhaseIncrement = nMidiPhaseIncrement[ulInput>>3]; // set the phase increment for grains 1 and 2, we do not want a precise frequency for these, // they set the character of the note rather than pitch ulGrain1PhaseIncrement = analogRead(1)<<18; ulGrain2PhaseIncrement = analogRead(2)<<18;}void TC4_Handler(){ // We need to get the status to clear it and allow the interrupt to fire again TC_GetStatus(TC1, 1); ulPhaseAccumulator += ulPhaseIncrement; // 32 bit phase increment, see below // if the phase accumulator over flows - we have been through one cycle at the current pitch, // now we need to reset the grains ready for our next cycle if(ulPhaseAccumulator < ulPhaseIncrement) { ulGrain1PhaseAccumulator = 0; ulGrain2PhaseAccumulator = 0; } else { // increment the grains ulGrain1PhaseAccumulator += ulGrain1PhaseIncrement; ulGrain2PhaseAccumulator += ulGrain2PhaseIncrement; } // mix the grains by adding them together - to the result of adding these two 32bit values into the 12 bit ADC Output // we shift both values right 21 places to sum two 11 bit values giving a 12 bit result. uint32_t ulOutput = (ulGrain1PhaseAccumulator>>21)+(ulGrain2PhaseAccumulator>>21); // we cheated and user analogWrite to enable the dac, but here we want to be fast so // write directly dacc_write_conversion_data(DACC_INTERFACE, ulOutput);}

Saturday, November 24, 2012

1) External Audio - There is now an option to use a very simple amplifier circuit which is small enough to fit inside most project boxes. This will give you a lot more volume when you are using the system outdoors.

2) Countdown - With countdown switched on, the lap timer will beep to count down the last few seconds of the current lap time - it adds a little extra pressure - to beat the lap you have to beat the beeps.

Before we get to the build, here are some user pictures of current builds, if you have pictures of your own build I am happy to share them.

ALLY - Transponder mounted in touring car chassis

Howie314 - Using an LCD Shield for a fast build

This build by Arduino Forum User Howie314 uses a LCD Keypad Shield, it keeps the size down and means there is very little soldering required to build the project. Many LCD Shields are available and all work in a similar manner meaning that you can get a lap timer up and running quickly with almost no soldering required.

If someone would design an LCD Shield with a rotary encoder as well as buttons we could have the perfect lap timer interface.

Howie314 is one of the first using the most recent version of the code with the new options of external audio and countdown. Additional options that I am considering in the near future are 1) Window magnet transponders and 2) AIM Transponders. See the end of post for more details.

Howie also did some work on the user interface as the LCD Shield uses a single analog pin to read multiple buttons. It makes sense to offer a version of the build along based on these readily available shields so I will be taking a look at Howies modifications with a plan to offer a single version of the project which can be run on standalone builds or LCD Shields - props to Howie.

ALLY's Build and Enclosure

Arduino forum user ALLY has used a standalone LCD and soldered his own buttons. This gives a bit more flexibility in the type of display and enclosures you can use, but also requires more work to build.

ALLY Transponder Mount

Here is a shot of ALLY's transponder mounted in his RC Touring car. Notice that the IR Emitter is mounted on a small post, this is a nice solution as the LED is well away from any areas that would impact in a crash.

If you want to try something similar make sure to keep the wires to the LED short. I tried wires of about 15 cm which unfortunately acted as an antenna causing radio interference.

ALLY's Carbon

ALLY has used a large carbon fibre effect enclosure, there is lots of room to add indicator LEDs, or one of my favorite features - a small amplifier for optional external speakers.

Lap Timer Piezo Buzzer and External Audio
One of the most useful features of the lap timer is instant audio feedback, this is particularly important with RC Car racing and Kart racing where corners are fractions of a second apart and checking a display is not an option.

The feedback is deliberately simple but totally effective - one beep to confirm a lap and two beeps if its a new best lap.

To add the audio features we have two options

1) A Piezo Buzzer
2) An External Speaker

I have found it useful to have both options in my build, using the quieter peizo for testing near my home and the external audio to overcome the additional noise at the track, the latest code includes a menu to switch between these two output options.

Piezo With Transistor Driver
To get as much volume as possible from the buzzer we can drive it using the battery voltage rather than the regulated 5 volts from the Arduino. To switch the higher battery voltage from an Arduino pin we use a NPN transistor connected between the piezo (the load) and ground. This arrangement allows us to switch the higher voltages and currents through the Peizo and can also be applied to other loads such as high powered LEDs, relays and other components you may want to use in your future projects.

Components
1 * 2.2K Resistor1 * P2N2222 Transistor1 * Piezo Buzzer

External Audio Through A SpeakerThe Peizo option can be used without adding the external audio option, but if you want more volume in your build read on -

In order to power a speaker we need a few more components to drive the extra current, one easy to use option is the LM386 Audio Amp chip. This is used extensively throughout the RCArduino blog in a variety of audio projects, full details and videos of the chip in action cab be found here -

Schematic - Note the audio output is from analog pin 5, for simplicity only the audio components are shown, as there

For the latest code, contact me Duane B though the Arduino forum.

Future Developments
Some common requests that I will be adding in the near future are -

1) Support for individual transponders - this will take some experimenting, but it should be possible to create a simple transponder scheme which will allow several lap timers to be used simultaneously with each car paired to a specific lap timer.

2) Support for magnetic transponders - I am told that many kart tracks use a magnetic strip under the track to activate transponders which are similar to magnetic window sensors, I hope to work with an Arduino forum user to add support for this option, it should be simple, but I have nowhere to test the system locally.

Inspired by Howie314's use of an off the shelf LCD Shield I will also add support for a build along based on this quick start option.

Thursday, November 15, 2012

This is the first in a series of posts introducing simple audio effects that can be used in micro controller projects.

Next Week - Bit Crushing effects

The Delay effect is one of the simplest and most effective enhancements we can add to our audio projects.

The delay effect works by recording the output as it is being generated and then mixing this sound back in with itself - after a delay. The result should be familiar to anyone who has every played an electric guitar through an amp with reverb.

In the case of the Auduino synthesizer the result is a mild echo effect and slightly smoother, more metallic sound - the effect can be turned on or off through a push button in the code provided below.

How do we create the delay effect
Delay is very simple to add in a microcontroller, all we need is a block of memory to record the output in.

The larger the block of memory, the longer the delay we can record and the deeper the effect.

In this case we are using a 1K block of memory in the array named sDelayBuffer -

// Duane B
// rcarduino.blogspot.com
// 15/11/2012
// Very simple ring buffer delay
// we record the output in this array
// and then mix it back in with the output as the buffer wraps around
// can be switched on and off by a button on DELAY_BUTTON
#define MAX_DELAY 1024
unsigned char sDelayBuffer[MAX_DELAY];
unsigned int nDelayCounter = 0;
unsigned char bDelay;

The other modification is inside the interrupt service routine which generates the Audiuno output, essentially what we are doing is adding the sound we recorded 1/8th of a second ago on top of the current output value -

// Duane B
// rcarduino.blogspot.com
// 15/11/2012
// add a button to set bDelay true or false to turn delay on and off
if(bDelay)
{
// Output to PWM (this is faster than using analogWrite)
// Here we add the delay buffer to the output value, this produces
// an subtle echo effect, the delay buffer is effectivley replaying the sound from
// 1/8th of a second ago.

// add the new output to the buffer so we can use it when the buffer next wraps around
sDelayBuffer[nDelayCounter] = PWM_VALUE;
nDelayCounter++;
if(nDelayCounter == MAX_DELAY)
{
nDelayCounter = 0;
}
}
else
{
LED_PORT &= ~(1 << LED_BIT); // Faster than using digitalWrite

PWM_VALUE = output;
}

We test whether delay is enabled, if it is we calculate the output value by adding the initial output to the earlier recorded output from our delay buffer. After outputting this combined value we record it in the delay buffer replacing the value we just used. Over time, the code cycles through the delay buffer over and over again, mixing the current output with a sample from 1/8th of a second back - a bit like playing your instrument in a large hall where the distinct sound is the result of the current sound being constantly mixed with its echo.

Thats all there is to generating delay in a micro controller synth engine - exactly the same code is used to create the delay effect in the RCArduino Five Dollar Synthesizer.

The RCArduino Five Dollar Synthesizer is another audio project enhanced with this delay effect -

Further Development
The amount of delay we can provide is determined to the memory we use to record the samples. In the Auduino we are using 1K which at an 8K play back rate gives us 125ms of delay. This can be increased by bit crushing the samples - using 4 bits per sample we get 250ms, 2 bits gets us half a second, with 1 bit we can get a whole second. Unfortunately initial experiments suggest that the effect is largely lost when applying these techniques, its a bit like shouting into a cave and getting a different echo back - your ears just don't buy it.

Auduino with delay is a very slight modification by Duane B (rcarduino) to the original work of Peter Knight.

Notes
- This code also include the volatile fix which allows the Auduino to work correctly in Arduino 1.0 and later
- Remember to use a pull up or pull down resistor if you are not using a push button or switch for the delay button or if your more comfortable modifying the code, replace the button code with true or false.
- LED 13, now indicates whether delay is on or off.

// Duane B
// rcarduino.blogspot.com
// 15/11/2012
// Very simple ring buffer delay
// we record the output in this array
// and then mix it back in with the output as the buffer wraps around
// can be switched on and off by a button on DELAY_BUTTON
#define MAX_DELAY 1024
unsigned char sDelayBuffer[MAX_DELAY];
unsigned int nDelayCounter = 0;
unsigned char bDelay;

// set pin mode and turn on pull up so that default mode
// is PENTATONIC, pull the pin low to switch to smooth
pinMode(SMOOTH_PIN,INPUT);
digitalWrite(SMOOTH_PIN,HIGH);
}

void loop() {
// The loop is pretty simple - it just updates the parameters for the oscillators.
//
// Avoid using any functions that make extensive use of interrupts, or turn interrupts off.
// They will cause clicks and poops in the audio.

// updated 29/01/2013
// pull the DELAY_BUTTON pin high for delay, low for no delay
// use either a pull up/pull down resistor
// or a pull up resistor with a toggle switch between the pin and ground
bDelay = digitalRead(DELAY_BUTTON);

// Duane B
// rcarduino.blogspot.com
// 15/11/2012
// add a button to set bDelay true or false to turn delay on and off
if(bDelay)
{
// Output to PWM (this is faster than using analogWrite)
// Here we add the delay buffer to the output value, this produces
// an subtle echo effect, the delay buffer is effectivley replaying the sound from
// 1/8th of a second ago.

// add the new output to the buffer so we can use it when the buffer next wraps around
sDelayBuffer[nDelayCounter] = PWM_VALUE;
nDelayCounter++;
if(nDelayCounter == MAX_DELAY)
{
nDelayCounter = 0;
}
}
else
{
LED_PORT &= ~(1 << LED_BIT); // Faster than using digitalWrite

Sunday, November 11, 2012

Many RC Transmitters and Receivers provide access to the PPM Stream, this is a single stream of pulses which includes the information for all of the receiver channels in a single connection.

This stream can often be accessed by removing the receiver case and adding a single wire to an existing connection within the receiver.

If we can gain access to this stream we can write smaller, faster code and only need a single interrupt pin to read all of the receiver channels.

Using a single built in interrupt to read multiple channels is much faster than using pin change interrupts which leads to a visibly smoother output signal for your servos and ESCs.

This post concludes with a library which can be used to read a PPM stream using a single interrupt. The same library is also able to output upto 18 servo signals from a single Arduino UNO with no additional components. This is an increase of 6 servos over the standard library - its also faster leading to fewer glitches.

Scroll down for a video of the library and an RC Receiver hack in action -

What does the PPM Stream Look Like ?

The stream is made up of a series of short pulses, the first pulse is the start marker. The time between the start marker and the start of the next pulse defines the pulse length for channel one. The time to the next pulse defines the pulse length for channel 2 and so on. The end of the pulse stream is marked by a gap know as the frame space. This gap indicates that there are no more channels to receive and the next pulse will be the start of a new frame. Each frame contains the pulse widths for all of your receiver channels.

Note - unlike servo signals, in a PPM Stream it is the gaps between pulses that defines the pulse width for a channel, not the duration of the pulse itself which can be very short.

How do we find the PPM Stream ?
If your equipment provides direct access to the PPM Stream, skip over this part, if it does not, read on.

The PPM Stream is transmitted between your transmitter and receiver as a single data stream. Inside the receiver this signal is de-multiplexed to produce the individual channels signals as seen in the diagram.

Fortunately for us, the de-multiplexer is most often just a simple shift register clocked by the PPM Stream.

The shift register is clearly visible inside this Hitec HFS-03MM Receiver - its the IC in the center.

The PPM Stream is routed to the clock pin (clock A) of the shift register, the PWM Streams for the individual channels are taken from the shift register outputs (Q1a,Q2a,Q3a).

Usually we would try to read these outputs using three separate interrupt pins, however as they are directly derived from the clock signal we can access the same information by reading the (PPM) clock signal directly. This saves us two interrupt pins and a lot of code and memory.

The best bit - read on and theres a ready made library at the end of the post.

Breaking out the PPM Signal
As above, if your receiver already provides access to the PPM Stream, skip ahead, if not, here are two receivers I have hacked. Its as simple as follows -
To access the PPM Stream from Arduino we need to solder an additional wire to the clock pin of the 4015 shift register.

The Same 4015 Shift Register inside tapped for PPM Output using thin white wire soldered to the clock pin.

A male jumper wire attached to the hacked Hitec receiver ready for connection to Arduino -

VIDEO - The RC Arduino Library and Receiver Hack in action

Schematic showing connections in the video above

Multiplexing Servos From Arduino Using PPM Style Signals

In a recent RCArduino Post we showed how a 4017 Decade counter IC could be used to control 10 servos from a single Adruino pin. Each time the counter is clocked by the Arduino it sets one servo output low and sets the next one high. We control the clock pulses to control the pulse widths for the individual servo outputs. Demulitplexing a PPM signal is essentially the same process, the RC Receiver applies a clock pulse to a shift register which shifts the pulse from one output to the next, the longer between clock pulses, the longer the servo pulse.

How do we read this RC Receiver PPM Pulse stream with a micro controller ?

Now that we have access to the PPM Stream, how do we read it with our Arduino ?

First of all we need to synchronise with the pulse stream, we do this by waiting for the long pause (the frame space) which indicates the end of one frame and the start of the next.

1) Once we have found this space, we can set our channel counter to 0 and record the current time.

2 ) The next pulse that arrives will indicate the end of the channel 1 pulse. We calculate the channel one pulse width by subtract the time recorded in 1) above from the current time. We also store the current time as the starting point for the channel 2 pulse width

3) We repeat the above process - subtract the last pulse time from the current time to get the pulse width for each of the remaining channels

4) When we have received all of the channels we expect, we start again at 1.

At each stage of the process 1-4 we know whether we are expecting a channel signal or a frame space and so we can use this information to confirm synchronization with the PPM Stream.

Sample Arduino Code For Reading RC Receiver PPM Signal

The following code is taken from the RCArduinoFastLib -// we could save a few micros by writting this directly in the signal handler rather than using attach interrupt
void CRCArduinoPPMChannels::INT0ISR()
{
// only ever called for rising edges, so no need to check the pin state

// calculate the interval between this pulse and the last one we received which is recorded in m_unChannelRiseTime
uint16_t ulInterval = TCNT1 - m_unChannelRiseTime;

// if all of the channels have been received we should be expecting the frame space next, lets check it
if(m_sCurrentInputChannel == RC_CHANNEL_IN_COUNT)
{
// we have received all the channels we wanted, this should be the frame space
if(ulInterval < MINIMUM_FRAME_SPACE)
{
// it was not so we need to resynch
forceResynch();
}
else
{
// it was the frame space, next interval will be channel 0
m_sCurrentInputChannel = 0;
}
}
else
{
// if we were expecting a channel, but found a space instead, we need to resynch
if(ulInterval > MAXIMUM_PULSE_SPACE)
{
forceResynch();
}
else
{
// its a good signal, lets record it and move onto the next channel
m_unChannelSignalIn[m_sCurrentInputChannel++] = ulInterval;
}
}
// record the current time
m_unChannelRiseTime = TCNT1;
}

Reading the PPM stream with the RCArduinoFastLib

One of the main features of the RCArduinoFastLib is a servo library, why do we need another servo library when the existing servo library is well know, widely used and reliable ?

The standard Arduino Servo library has one major flaw - it resets timer1 at the start of every frame. This means that timer1 cannot be used for timing as easily as you might want. A common approach to overcoming this is to use the micros() function for timing, but this is many times slower than accessing TCNT1 directly.

As our channel input timing and servo output timing is interrupt driven, we really care about speed, every little bit of inefficiency leads to more and bigger interrupt clashes which introduce the ticks and jitter often seen in Arduino based RC projects.

By using the RCArduinoFastServos library we avoid resetting timer1, gain an additional six servos and also the added benefit of being able to user timer1 for very fast timing of our input signals. These lead to a measurable increase in output signal quality with fewer and smaller clashes in short - glitch free projects.